The invention relates to microwave circuits in general, and more particularly to a microwave circuit which combines a number of ferrite-type signal-to-noise enhancers to extend the signal-to-noise enhancement frequency bandwidth beyond that which is offered individually by any one of the enhancers being combined.
Just recently, completely passive two-port signal-to-noise enhancers have been developed utilizing the basic mechanism of exciting magnetostatic waves in ferrite-type material either in the form of a plurality of spheres or in the form of a film. An example of this type enhancer is disclosed in the copending U.S. application bearing Ser. No. 061,537, filed July 27, 1979 by John D. Adam and entitled "Magnetostatic Wave Signal-To-Noise Enhancer". In this example, an external bias field applied to the enhancer may be adjusted to select an operational frequency bandwidth in the frequency range over which the enhancer may be effectively excited to cause signal enhancement. Generally, enhancers of this type may be excited only over a limited frequency bandwidth. In the microwave frequency range of 2-5 GHz, for example, dependent upon the given ferrite material and the given bias field conditions, the effective operating frequency bandwidth may only be on the order of one-third octave. However, it is understood that by adjusting the bias field applied across the enhancer device, for example, it may be possible for the enhancer to cover different one-third octave segments over a given microwave frequency range.
Unfortunately, for some intended applications of this type enhancer, like in the area of electronic counter measures ECM, for example, it is desirable to cover frequency bandwidths on the order of an octave in the 2-5 GHz range. In these cases, in order to improve signal quality by enhancing the signal-to-noise ratio thereby, a system of more than one of the aforementioned type enhancers would be needed to cover the frequency bandwidth of all of the potential threat signals anticipated.
The parameters which predominantly limit the frequency response bandwidth of the enhancer are the saturation magnetization of the ferrite material utilized therein and the externally applied field strength. If a strong input signal has sufficient energy within the characteristic spin wave line width of the ferrite material and exceeds a power threshold level, it is passed relatively unattenuated through the enhancer. However, if the same amount of energy from an input signal is spread over a wide band of frequencies, the energy within the spin wave line width characteristic of the enhancer may not have sufficient strength to saturate the enhancer and will lose much of its power through insertion losses within the enhancer. Apparently, it is not enough for the input signals to fall within the responsive frequency bandwidth of the enhancer. They must also be coherent in nature or, in essence, have enough energy per unit bandwidth to cause excitation of the enhancer. In other words, the input signal power must exceed the characteristic power threshold level of the enhancer within the responsive frequency bandwidth thereof. Thus, the purpose of the enhancer is to prevent signals which fall in the characteristic frequency response bandwidth thereof and which have an energy content below a given power threshold level from passing relatively unattenuated therethrough. However, unlike amplifiers, input signals falling outside the responsive bandwidth of these enhancers, receive relatively low insertion losses in passing therethrough.
Attempts have been made to combine enhancers of the aforementioned type directly in cascade to achieve a greater enhancement frequency bandwidth. One problem which was observed from this configuration is that while the enhancers pass the signals from input to output which are outside of the bandwidth of the device, they do not pass them well. These enhancers act more like limiters outside of their characteristic response bandwidths. It is basically unpredictable what may happen to the signals passing through an enhancer of the aforementioned type having frequency content outside of the characteristic response bandwidth thereof; however, generally some insertion loss is evidenced. In some cases, this insertion loss may be high enough so that after the input signal passes through the first enhancer in cascade, there would be insufficient energy per unit bandwidth left in the signal by the time it got to the second enhancer in cascade, and so on, to operate them successfully, i.e., to excite the magnetostatic wave characteristics of the ferrite material. Therefore, a cascadedly-coupled enhancer configuration for purposes of extending the enhancement bandwidth thereof appears to be operationally ineffective for these purposes.
An alternative direct parallel configuration of the enhancers was also given consideration. However, in analysis, it appears that the energy of the input signals outside of the characteristic response bandwidth would pass through the enhancers with low insertion losses. Ostensibly, in a parallel configuration of the enhancers, some input signals may tend to leak around those enhancers specified to attenuate them and go through with relative low insertion losses those enhancers which were not designed to offer attenuation. Consequently, problems associated with mixing of signals, interaction and possibly intermodulation of signals are anticipated and the resulting signals would be entirely unpredictable.
It is therefore an object of the disclosure found hereinbelow to offer a microwave circuit for combining a plurality of the aforementioned type enhancers to extend their overall signal-to-noise enhancement frequency band-width to one more suitable and desirable for applications such as electronic countermeasures, for example.